The present invention relates to an operative temperature sensing system and more particularly to such a system that accurately, simply and inexpensively senses (for indication and/or control purposes) the operative temperature which closely approximates the thermal environmental sensing capabilities of a human being. Operative temperature is the uniform temperature of a radiantly black enclosure in which an occupant would exchange the same amount of heat by radiation plus convection, as in the actual nonuniform environment.
By way of background, a person generates or loses heat by conduction, convection and radiation. When a person is thermally neutral, the heat loss from the person equals the heat gain, and when this situation exists, a person is comfortable. Therefore, when there is a knowledge of what the operative temperature should be, a person's environment can be adjusted so that the above-expressed equilibrium is maintained. In order to adjust the environment there are certain factors which have to be taken into consideration, namely, the radiant convection and conduction heat transfer by which a person is losing or gaining heat or, in other words, the heat transfer between a human body and its environment.
Considering the foregoing still further, a heating, ventilating and cooling system has two purposes. The first is to maintain a person's comfort during the time that a room is occupied. The second is to maintain the integrity of the materials of a structure during unoccupied time. This means that all that is necessary when a room is unoccupied is to maintain the air temperature therein sufficiently high so that there is no damage due to freezing, or the like or, to maintain the temperature sufficiently low so that there is no mildewing. For example, an unoccupied room should normally not be maintained at an air temperature below 13.degree. C. and obviously should not exceed an air temperature of about 35.degree. C. Thus for optimum heating and cooling efficiency, for example, when a room is unoccupied in cold weather, the room can be at an air temperature of 13.degree. C. However, as soon as the room becomes occupied, the temperature should be raised to an acceptable operative temperature of approximately 20.degree. C. for comfort. Thus, when a room is unoccupied and maintained at 13.degree. C., heat loss is maintained at a practical minimum. However, once it becomes occupied, it is desirable to raise the temperature to 20.degree. C. operative temperature, for example, within a 15-20 minute period.
In existing heating systems utilizing hot air or hot water, the air temperature is raised by convection. However, human comfort can be realized in a much shorter time period by the use of radiant heat transfer alone, or in combination with the foregoing type of heat. Thus, when a person enters a room shortly after a radiation heating system has been energized, the person will feel the comfort from the radiant heat almost immediately, and the air temperature will be increased incidental to the radiant heating until both are of the required value to both maintain human comfort and also warm the various structures, such as tables, desks and chairs, with which humans come into contact. Thus, when radiant heat is used, there is practically no time delay in achieving human comfort even though the room temperature was well below the comfort level immediately before the radiant heat source was energized.
In U.S. Pat. No. 4,433,923 dated Feb. 28, 1984 and assigned to the present inventor, an operative temperature sensing system is disclosed which is based on the measurement of globe temperature and air temperature and which provides an output for control purposes. However, in the operative temperature sensing system which relies on the globe temperature, the operative temperature is expressed by the equation: EQU T.sub.o =AT.sub.g +(1-A)T.sub.a
In the above equation, T.sub.o is the operative temperature, T.sub.g is the globe temperature, T.sub.a is the air temperature, and A is a weighing factor. The weighing factor for a globe thermometer of 11/2 inch diameter has been calculated as 1.28 and thus in the patent the equation for operative temperature becomes: EQU T.sub.o =1.28T.sub.g -0.28T.sub.a
It has been found, however, that there is an inherent problem in using the output of a globe thermometer (T.sub.g) to calculate T.sub.o in the above equation. Under certain circumstances, because of its inherent long time constant in relation to T.sub.a, it causes the measured T.sub.o to produce an error signal when none is necessary because the T.sub.a response is faster than the T.sub.g response. From the above formula it is seen that the measured T.sub.o will decrease if T.sub.a increases while T.sub.g does not increase as rapidly in the heating mode, and it will increase if T.sub.a decreases and T.sub.g does not decrease as rapidly in the cooling mode. However, the foregoing calculation of T.sub.o in the above formula is directly opposite to what is desired because the actual air temperature is already higher than desired in the heating mode and its lower than desired in the cooling mode. Thus, when the erroneous T.sub.o calculated from the above equation is matched against a predetermined set point temperature, as expressed in the above patent, the signal will call for more heat when none is required and will call for more cooling when none is required. Thus there is a problem in using the above equation for providing an output of operative temperature which is matched against a predetermined set point temperature.